Fuel Cell System Design

(1)

Fuel Cell System Design

(2)

How to Design A FC System?

1. Construct a reasonable system configuration and make a good guess on the specifications.

2. Calculate the thermal and mass balance of the complete system based.

3. Refine the choice and specification of system components

according to the thermal and mass balance calculated in 2. For according to the thermal and mass balance calculated in 2. For coupled components, verify compatibility based on the

expected magnitude and rate of mass, heat or current transfer between them

between them.

4. Review the system’s performance considering the original design goals If system refinement is required decide which design goals. If system refinement is required, decide which components or parameters should be changed and repeat the design process.

(3)

20 W (12 V)Portable SOFC Schematic

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SOFC System Components

1. Fuel cell stack:

• Should supply net power output of 20 WShould supply net power output of 20 W.

• Obviously, stack power is significantly larger than 20W.

• Support BOP’s such as the air blower and the DC/DC converter

P t 12 V (17 ll t 0 7 V lt ti l

• Power at 12 V (17 cells at 0.7 V or alternatively fewer cells with a 12 V DC-DC converter)

2. Hydrogen supply

• Use metal hydrides for simplicity in a portable system

system.

• Relatively high pressures/flow rates

• Hydrocarbon-fuel provide better energy density, y p gy y, but complicated

(5)

SOFC System Components

3. Air Supply:

• For electricity and thermal management (cooling)For electricity and thermal management (cooling)

• Relatively large flow rates will be needed for cooling

• Options: compressor, fan, or blower

• Choose blower working at 12 V DC 4. Heat management

• Recycle heat from the stack

• Heat exchanger can warm up the cold fuel cell inlet gases using the hot fuel cell exhaust gases.

(6)

SOFC System Components

5. DC-DC converter

• 6~8 cells generates 4 2~5 6V at 0 7V6 8 cells generates 4.2 5.6V at 0.7V.

• DC-DC converter to boost up the output voltage to 12 V sacrificing efficienty.

• Stabilizes stack voltage fluctuation

(7)

SOFC Unit Cell Performance

Q1: How to get this?

Q2: Operating point? (voltage, cell temerpature, t i hi t ffi i

stoichiometry, efficiency Q3: How many stacks?

(8)

System Design = Chicken and Egg

Stack power = f(power density)

Power density of SOFC = f(operating temperature) Operating temperature = f(cooling flow rate, say air) Flow rate f(blower power)

Flow rate = f(blower power) Blower power = f(stack power)

-The various parameters in the system are strongly coupled nonlinearly

Solve thorough initial guess and iteration - Solve thorough initial guess and iteration

(9)

Initial Guess on Stack and BOPs

Stack design parameters Value

F l ll i T 00 C

Fuel cell operating temperature, T_fc 700 C Hydrogen and air pressure 1 atm Hydrogen stoichiometry, 1.2

Air stoichiometry, y, 8

Fuel cell operating voltage, V_oper 0.7 V

Fuel cell output power P 50W

Fuel cell output power, P_fc 50W System component design parameters Value System component design parameters Value DC-DC converter efficiency, 90%

Heat exchanger efficiency, 90%

(10)

Bookkeeping: 1, Mass Balance

A_fc = 47.62 cm² = 50 W / (0.7 V X 1.5A/cm²)) . i = 71 43 A (= 50 W / 0 7 V)

i_total = 71.43 A (= 50 W / 0.7 V).

2 supply 2

71.43 2 96400 / 1.2

total

H H

i A

v = F ×

λ

= C l ×

2,supply 2

4

2 96400 /

4.442 10 / 0.02665 / min

H H

nF C mol

mol s mol

−

×

= × =

2,supply 2

71.42 4 96400 / 8

total

O O

i A

v = F ×

λ

= C l ×

2,supply 2

4 96400 /

0.001481 / 0.08884 / min

O O

nF C mol

mol s mol

×

= =

2 2

3

,supply ,supply

1.481 10 / 0.79

N O

0.21

v = v × =

ω

× ⁻ mol s×

0.

0.005571

mol s

/ 0.3342

mol

/ min

= =

(11)

Bookkeeping: 1, Mass Balance

2 2

,supply ,supply ,supply 0.3342 / min 0.08884 / min 0.4230 / min

Air N O

v = v +v = mol + mol = mol

,supply 25

,supply

0.4230 / min 0.0820578 / 298.15 1

C Air Air

v RT mol atm L mol K K

V p atm

× ⋅ ⋅ ×

= =

10.35LPM

=

71.42 4

3 702 10 / 0 0222 / min

total

i A

v = = = × ⁻ mol s = mol

2 , 3.702 10 / 0.0222 / min

2 96400 /

H O prod

v mol s mol

nF C mol

= = = × =

×

2 2 2

5

, ,supply ,

l

7.403 10 / 0 001296 /

H exhaust H H cons

O h O O

v v v mol s

= − = × −

= − =

2 2 2

2 2

, ,supply ,

, ,supply

0.001296 / 0.005571 /

O exhaust O O cons

N exhaust N

v v v mol s

v = v = mol s

2 2

4

, ,

3.702 10 /

H O exhaust H O prod

v = v = × ⁻ mol s

(12)

Bookkeeping: 1, Mass Balance

FLOW RATES VALUE (MOL/S)

v_H2,supply 4.442 x 10^-4

v_O2,supply 0.001481

v_N2 _l = v_N2 _h _t 0 005571

v_N2,supply v_N2,exhaust 0.005571

v_H2,exhaust 7.403 x 10^-5

0 001296

v_O2,exhaust 0.001296

vH2O,exhaust = v_H2,cons= 2v_O2,cons 3.702 x 10^-4

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Bookkeeping: 2, Thermal Balance

Assuming adiabatic P_heat = (E^H −V_oper)×i_total

(1.28V 0.7 ) 71.43V 41.71W

= − × =

Heat is only removed by convection

( )

, , , , , ,

heat p i i exhaust fc p i i exhaust fc out fc in

i i

P = ⎛⎜ c v ⎞⎟ΔT = ⎛⎜ c v ⎞⎟ T −T

⎝

∑

⎠ ⎝

∑

⎠

⎝ ⎠ ⎝ ⎠

( )

41.71

heat

H H h t O O h t N N h t H O H O h t f t f i

P W

c v c v c v c v T T

= =

⎡ + + + ⎤ −

⎣ ^, ₂ ₂^, ^, ₂ ₂^, ^, ₂ ₂^, ^, ₂ ₂ ^, ⎦

(

^, ^,

)

(30.116 / 0.00007403 / 34 366 / 0 001296 /

p H H exhaust p O O exhaust p N N exhaust p H O H O exhaust fc out fc in

c v c v c v c v T T

J mol K mol s

⎡ + + + ⎤

⎣ ⎦

= × +

× +

34.366 / 0.001296 / 32.409 / 0.005571 /

40 924 / 0

J mol K mol s J mol K mol s J mol K

× +

× ⁰⁰⁰³⁷^{mol s T}^{/ )}

(

^f ⁻^{973 15}

)

40.924 /J mol K ×0.

(

^,

)

,

00037 / ) 973.15

1145

fc out fc out

mol s T

T K

∴ =

(14)

Bookkeeping: 2, Thermal Balance

Heat exchanger ^HX ^{p i i}^, ^hot ^HX ^{p i i}^,

(

^{hot in}^, ^{hot out}^,

)

^HX

i i

Q = ⎛⎜ c v ⎞⎟ΔT ε = ⎛⎜ c v ⎞⎟ T −T ε

⎝

∑

⎠ ⎝

∑

⎠

( )

, , , ,

i i

p i i cold p i i cold in cold out

i i

c v T = c v T T

⎝ ⎠ ⎝ ⎠

⎛ ⎞ ⎛ ⎞

= ⎜ ⎟Δ ⎜ ⎟ −

⎝

∑

⎠ ⎝

∑

⎠

i i

⎝ ⎠ ⎝ ⎠

( )

⎡ , ₂ ₂, , ₂ ₂, , ₂ ₂, ⎤

(

, ,

)

(30.116 / 0.000444 /

HX p H H exhaust p O O exhaust p N N exhaust cold out cold in

Q c v c v c v T T

J mol K mol s

⎡ ⎤

= ⎣ + + ⎦ −

= × +

( )

34.366 / 0.001481 /

32.409 / 0.005571 / ) 973.15 298.15 J mol K mol s

J mol K mol s K K

× +

×

(

−

)

165.3W

=

(15)

Bookkeeping: 2, Thermal Balance

( )

165.3

HX

H H h t O O h t N N h t H O H O h t h t i h t t HX

Q W

c v c v c v c v T T ε

= =

⎡ + + + ⎤ −

⎣ ⎦

( )

2 2 2 2 2 2 2 2

, , , , , , , , , ,

(30.116 / 0.00007403 / 34.366 / 0.001296 /

p H H exhaust p O O exhaust p N N exhaust p H O H O exhaust hot in hot out HX

c v c v c v c v T T

J mol K mol s

ε

⎡ + + + ⎤

⎣ ⎦

= × +

× +

32.409 / 0.005571 / 40.924 /

J mol K mol s J mol

× +

(

^,

)

0.00037 / ) 1145 _{hot out} 0.9 K × mol s

(

K −T ^,

)

×

, 388 115

hot out

T K C

∴ = =

THERMAL PARAMETERS VALUES THERMAL PARAMETERS VALUES

P_Heat 41.7 W

165.3 W

T_fc,in= T_cold,out 973.15 K

T_fc,out= T_hot,in 1145 K

T_cold,in= T_ambient 298.15 K

T_cold,out 388K

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Component Selection

SPECIFICATION VALUE Max flow rate 15 5 LPM

Air Blower

Max flow rate 15.5 LPM Max current 3.2 A

Operating voltage 12 V Operating voltage 12 V

Max pressure 1200 mbar Power vs flow rate Linear ( ) Power vs. flow rate Linear ( )

Heat Exchangerg Specification^p Values Max fluid temperature 900°C

Rated flo range 5 20 LPM Rated flow range 5–20 LPM Number of paths 4

Thermal efficiency 90%

(17)

Component Selection

DC-DC converter SPECIFICATION VALUES

Input voltage Min 4.0 V, Max 25 V Output voltage (Adjustable) Min 1.5, Max 15 V Max input current 20 A

Max output current 5 A

Metal hydride

Efficiency 90%

SPECIFICATION VALUES

y SPECIFICATION VALUES

Dimension D 6.4cm, H 26.5cm

Weight 2 2 kg

Weight 2.2 kg

Hydrogen capacity 250 L 1

Internal pressure 17 atm

(18)

System Characteristic

N t P P P

Net power Blower

net fc DC DC blower

P = P ×

ε

₋ − P

Max current

Actual flow rate

P × ×V

Blower

power Actual flow rate

Max flow rate

3.2 10 35 12 25 64

blower blower

P V

A LPM V W

= × ×

= × × =

N t

10.35 12 25.64

15.5 LPM V W

= LPM × × =

50 0 9 25 64 19 36 P = W × W = W Net power

Efficiency

50 0.9 25.64 19.36 Pnet = W × − W = W

net net

net

P P

h h

ε = =

Δ Δ

Efficiency

2,supply

19.36 19.36

.176

H 110

HHV H

HHV h v

h

W W

l J

Δ ×

Δ

= = =

4 2

2

H 110

247, 700 J 4.442 10 mol W

mol H s

× ∗ −

(19)

Design Review

SPECIFICATION VALUE

System net power 19.35 W (Need to be increseas)

Fuel cell power 50 W

Fuel cell voltage 4.2 V (Maybe too low)

Number of cells in the fuel cell stack 6 (Need to be increase)( ) Temperature range of the fuel cell 700~872°C (T_diff too high) Temperature range of the heat exchanger 25~872°C (T_diff larege) System operation time 6 4 hour (Too short)

System operation time 6.4 hour (Too short)

Air blower power consumption 25.64 W (More than 50%)

-Heat dissipation to the environmentHeat dissipation to the environment -Increase air flow? Blower power?

-Increase cell number? How about cost?

-Pressure drop in the stack?

-Weight, volume and cost?

Cell performance change in different operation condition?

-Cell performance change in different operation condition?

-Single cell performance vs stack performance?